1. Field of the Invention
The present invention relates to a semiconductor laser device and a process for manufacturing the same. More particularly, it relates to a forward embedded mesa ridge semiconductor laser device provided with a forward mesa ridge on an upper cladding layer and a second upper cladding layer on the surface of the forward mesa ridge to reduce the resistance of the device and stabilize the transverse mode characteristic of a laser beam produced by the laser, and a process for manufacturing the laser.
2. Description of the Relate Art
As the storage capacities of information recording media used for personal computers and multimedia appliances increase, demands have been increasing for MO (magneto-optical disk) and CD-R (field progranmmable compact disk) that are information recording media wherein information can be written by the use of light. For laser devices used in writing information on such programmable optical disks, an infrared emitting laser beam having a wavelength of 780 nm and red laser beams having wavelengths in a range from 650 nm to 685 nm are used, which require increased output power while ensuring a stable transverse mode oscillation laser beam.
FIG. 8 is a perspective view of a forward mesa ridge-embedding type semiconductor laser of the prior art, and FIG. 9 is a cross sectional view taken along lines IX--IX in FIG. 8.
In FIG. 8 and FIG. 9, numeral 100 denotes the forward mesa ridge-embedding type semiconductor laser device, 102 denotes an n-type GaAs substrate having a (100) principal plane, 104 denotes a lower cladding layer made of n-type Al.sub.0.5 Ga.sub.0.5 As disposed on the substrate 102, and 106 denotes an active layer having a multiple quantum-well (hereinafter referred to as MQW) structure disposed on the lower cladding layer 104.
The MQW structure comprises wells or well layers made of undoped Al.sub.0.1 Ga.sub.0.9 As, guide layers, and barrier layers (outermost layers) which are made of undoped Al.sub.0.3 Ga.sub.0.7 As (hereinafter the MQW of this constitution will be referred to as an undoped Al.sub.0.3 Ga.sub.0.7 As/Al.sub.0.1 Ga.sub.0.9 As MQW. MQWs made of materials of other constitutions will also be referred to similarly.)
Numeral 108 denotes an upper cladding layer made of p-type Al.sub.0.5 Ga.sub.0.5 As, 108a denotes a forward mesa ridge of the upper cladding layer 108, 108b denotes a parallel portion of the upper cladding layer 108, 110 denotes a cap layer made of p-type GaAs disposed on the forward mesa ridge 108a, while the forward mesa ridge 108a and the cap layer 110 constitute the forward mesa ridge portion 109.
Numeral 112 denotes a current blocking layer made of n-type GaAs disposed on the parallel portion 108b of the upper cladding layer 108, embedding the cap layer 110 and the forward mesa ridge 180a. Numeral 114 denotes a contact layer made of p-type GaAs disposed on the cap layer 110 and on the current blocking layer 112 in contact therewith. Numeral 116 denotes a p-electrode and 118 denotes an n-electrode.
Numeral 120 denotes a beam emimtting end face, 122 denotes a back end face of the semiconductor laser 100, and arrow 124 represents a laser beam.
FIG. 9 is a cross sectional view of the forward mesa ridge-embedding type semiconductor laser without the p-electrode 116 and the n-electrode 118.
The semiconductor laser 100 of such a configuration as described above operates as follows.
When a forward bias voltage is applied between the p-electrode 116 and the n-electrode 118, holes and electrons are supplied from the p-electrode 116 and from the n-electrode 118 respectively, to the active layer 106 through a path which is limited by the current blocking layer 112. The carriers (electrons and holes) are confined in the active layer 106 by the lower cladding layer 104 and the upper cladding layer 108, so that spontaneous emission of light occurs through recombination of electrons and holes in the active layer 106. The spontaneously emitted light is confined in the forward mesa ridge portion 108a that serves as a waveguide between the lower cladding layer 104 and the upper cladding layer 108, being reflected by the beam emitting end face 120 and the back end face 122, thereby inducing stimulated emission and providing an output of the laser beam 124.
In the semiconductor laser 100, the active layer is an undoped Al.sub.0.3 Ga.sub.0.7 As/Al.sub.0.1 Ga.sub.0.9 As MQW structure while the lower cladding layer 104 and the upper cladding layer 108 have compositions of Al.sub.0.5 Ga.sub.0.5 As, and therefore the lower cladding layer 104 and the upper cladding layer 108 have a larger bandgap and a lower refractive index than the active layer 106. The current blocking layer 112 is made of GaAs, and therefore has a smaller bandgap and higher refractive index than the upper cladding layer 108.
Consequently, the light generated in the active layer 106 beneath the forward mesa ridge portion 108a of the upper cladding layer 108 is confined in a region defined between the forward mesa ridge portion 108a of the upper cladding layer 108 and the lower cladding layer 104 in the vertical direction.
Transverse light is confined by forming the parallel portion 108b of the upper cladding layer 108 with a predetermined thickness so that the active layer 106 is adjacent to the current blocking layer 112, and by having a certain part of light absorbed by the current blocking layer 112, making use of the difference in the bandgap between the active layer 106 and the current blocking layer 112 via the parallel portion 108b. This type of semiconductor laser device is called a gain waveguide semiconductor laser device.
Now a process for manufacturing the semiconductor laser device of the prior art will be described below.
The lower cladding layer 104 of n-type Al.sub.0.5 Ga.sub.0.5 As, the active layer 106 of an undoped Al.sub.0.3 Ga.sub.0.7 As/ Al.sub.0.1 Ga.sub.0.9 As MQW structure, the upper cladding layer 108 of p-type Al.sub.0.5 Ga.sub.0.5 As, and the cap layer 110 of p-type GaAs are formed successively on the n-type GaAs substrate 102 by an epitaxial growth process such as MOCVD.
Then an SiON film is grown on the cap layer 110 by CVD or the like to form a striped mask pattern through photolithography and etching steps, and top portions of the cap layer 110 and of the upper cladding layer 108 are removed by wet etching with the mask pattern used as a mask, thereby to form the forward mesa ridge portion 109 comprising the forward mesa ridge 108a and the cap layer 110.
Side faces of the forward mesa ridge 108a are defined by etching in a (111) A plane. Formed on the side faces of the forward mesa ridge portion 109 are current blocking layers 112 of n-type GaAs grown in the epitaxial growth process.
The mask pattern of SiON film is removed by wet etching or the like, and the contact layer 114 of p-type GaAs is formed by the MOCVD process on the cap layer 110 and on the current blocking layer 112.
Finally, the p-electrode 116 is formed on the contact layer 114 by vapor deposition or the like and, after polishing the back surface of the substrate 102 to a thickness of about 100 .mu.m, the n-electrode 118 is formed by vapor deposition or the like, thereby completing the semiconductor laser 100.
The reason for providing the cap layer 110 on the upper cladding layer 108 of the semiconductor laser device 100 is because, without the cap layer 110, the upper cladding layer 108 would be exposed to the atmosphere and oxidized when the etching mask is removed after forming the forward mesa ridge 108a, because the upper cladding layer 108 is made of Al.sub.0.5 Ga.sub.0.5 As.
When an oxide film of Al is formed, electrical resistance of the device increases since this portion lies in the current path, and deterioration occurs in the crystal after regrowth. In order to prevent such a problem, the cap layer 110 of p-type GaAs is formed in the first epitaxial growth thereby to prevent an oxide film from being formed on top of the forward mesa ridge 108a when removing the etching mask after forming the forward mesa ridge 108a.
However, since the bandgap of GaAs constituting the cap layer 110 and the contact layer 114 is smaller than that of the active layer, light is absorbed in the cap layer 110. Thus absorption loss of the laser beam is decreased by making the upper cladding layer 108 with sufficient thickness.
The traditional laser device, being constructed as described above, is subjected to such a restriction that the upper cladding layer 108 must have thickness not less than a predetermined minimum, for example, 1.5 m.
Transverse light confinement is achieved by having a certain portion of light absorbed by the current blocking layer 112 by making use of the difference in the bandgap between the active layer 106 and the current blocking layer 112 which adjoin each other via the parallel portion 108b. This leads to such a problem that the transverse mode of the laser beam becomes unstable unless the width w of the base portion of the forward mesa ridge 108a is limited within a predetermined maximum.
In order to stabilize the transverse mode of the laser beam, the base width w of the forward mesa ridge 108a must be, for example, about 4 .mu.m or less.
When the principal plane of the substrate 102 is taken in a (100) plane, however, the angle .alpha. formed between the normal direction of the parallel portion 108b of the upper cladding layer 108 and an (111)A plane which forms the side face of the forward mesa ridge 108a is 54 degrees, an angle that is defined by etching.
When the base width of the forward mesa ridge 108a is set to 4 .mu.m, the forward mesa ridge 108a has a height limit in case both side faces of the forward mesa ridge 108a intersect, namely width u of the forward mesa ridge 108a is zero, so that the height limit is determined geometrically, that is about 1.45 .mu.m high (the height limit will hereinafter be referred to as a "point height limit", while the thickness of the cap layer 110 is far less than the height of the forward mesa ridge 108a and is therefore negligible).
As described above, the upper cladding layer 108 must be about 1.5 .mu.m or thicker in order to suppress the absorption loss of light in the cap layer 110, and the parallel portion 108b normally has a thickness of 0.2 to 0.3 .mu.m. Even when this is taken into consideration, the angle formed between the normal directions of the parallel portion 108b of the upper cladding layer 108 and the side face of the forward mesa ridge 108a is predetermined, and therefore the height of the forward mesa ridge 108a becomes close to the "point height limit" with the top width u of the forward mesa ridge 108a becoming extremely narrow, resulting in increased resistance of the device.
FIG. 10 is a perspective view of a forward mesa ridge-embedding type semiconductor laser device of an another prior art. FIG. 11 is a cross sectional view taken along lines XI--XI in FIG. 10.
In FIG. 10, numeral 200 denotes the forward mesa ridge-embedding type semiconductor laser device employing an offset substrate. Numeral 202 denotes an n-type GaAs substrate having a principal plane offset by 10 degrees from a (100) plane toward a [011] orientation.
Numeral 204 denotes a lower cladding layer made of n-type Al.sub.035 Ga.sub.0.15 In.sub.0.5 P, 206 denotes an active layer having an undoped Al.sub.0.15 Ga.sub.0.35 In.sub.0.5 P/GaInP MQW structure, 208 denotes an upper cladding layer made of p-type Al.sub.0.35 Ga.sub.0.15 In.sub.0.5 P, 210 denotes a cap layer made of p-type GaAs, 212 denotes a current blocking layer made of n-type GaAs, 214 denotes a contact layer made of p-type GaAs, 216 denotes a p-electrode and 218 denotes an n-electrode.
The configuration of this prior art example is different from that of the prior art example described previously in that the substrate is offset and accordingly side faces of the forward mesa ridge 208a are not symmetrical.
When the offset substrate is used, the angle .beta. formed between the normal direction of the (111)A plane which forms one side face of the forward mesa ridge 208a and the parallel portion 208b of the upper cladding layer 208 becomes larger than 54 degrees, while the angle formed between the normal direction of the parallel portion 208b and the other side face becomes smaller.
Thus there has been such a problem that, when the ridge is made higher by making the upper cladding layer 208 thicker in order to suppress the absorption loss of light in the cap layer 210, the top width of the forward mesa ridge 208a becomes even smaller than the previous prior art example which employs the substrate 102 having a (100) plane surface, because the ridge is inclined, namely the side faces are asymmetrical, thus resulting in higher device resistance.
Also in the case of the forward mesa ridge 208a that employs the offset substrate, there has been a problem that, because the apex of the forward mesa ridge 208a is not located at the center of the forward mesa ridge 208a, non-uniform current injection into the active layer 206 as well as the narrower plateau of the forward mesa ridge 208a make it more difficult to stabilize the transverse mode.
The problems described above are not limited to the gain waveguide type semiconductor laser device, but are also encountered in a refractive index waveguide type semiconductor laser device having a current blocking layer made of a material having a bandgap larger than that of the upper cladding layer.